The present invention relates to a supply pump for fuel injection into an internal combustion engine, and more particularly, to a supply pump for maintaining high pressure in a common rail fuel injection system.
Direct gasoline injection has some distinct advantages regarding emissions and fuel economy mainly because it allows increased compression ratio of the engine (directly affecting the efficiency of the thermal cycle) without however requiring high octane (leaded) gasoline.
Many passenger car manufacturers are currently trying to develop such systems but one of the main obstacles is unavailability of a reliable and inexpensive pump capable of generating relatively high pressure. High pressure supply pumps currently under industry development for diesel common rail applications, could theoretically be easily modified for use in gasoline direct injection common rail systems. However, inherent to its design, such a pump would have some serious drawbacks because of all the compromises which would have to be made.
In order to prevent formation of vapor cavities in the pump housing (especially in the cam box), to handle variations in fuel quality (winter fuel) and to operate under any imaginable conditions (temperature and altitude), the pump housing must be always pressurized to at least about 2 bar.
The (electric) feed pump must be located either in the tank itself or in close proximity. On a hot summer day and with only partially filled tank (faster fuel recirculation), the fuel temperature in the tank can reach estimated levels of up to 140xc2x0 F. Because of low gasoline vapor pressure, the feed pump must be installed below the lowest expected fuel level in the tank, in order to ensure so called positive suction height.
Typical electric feed pumps used with conventional low pressure, mostly called indirect or also manifold gasoline injection, usually operate in the pressure range of about 3-4 bar. Such feed pressure is insufficient for use in a diesel supply pump adapted for gasoline pumping.
Considering the short charging duration of an intermittently operating cam and the higher speed range of gasoline engines, the absence of retraction assisted plunger/shoe/roller assembly motion reversal, and also the necessity to overcome the required higher housing pressure, the minimum pressure the feed pump must generate would have to be well above 7 bar, which is more or less the pressure limit of a typical fuel filter.
Because of a fire hazard danger in the case of even a small gasoline leak, all dynamic and stationary seals would have to be modified to ensure proper sealing of the higher pressure, and every seal would also have to be backed up by another redundant seal. This would lead to a substantial increase of overall dimensions of a diesel pump, which is already too big for the typically smaller gasoline engines.
At 120 bar pressure level the amount of the fuel stored in the rail by compressibility of fuel only and available for injection would be minimal. In order to maintain more or less constant rail pressure required for operation of an open loop controlled injector, either greater accumulator volume or some kind of accumulator assistance, would be necessary. However, the resulting lower xe2x80x9cspring ratexe2x80x9d of the accumulator would require further increase of the pump capacity in order to ensure satisfactory system dynamics (whether for an inlet metered or a waste gate controlled pump), resulting in many additional potential problems such as supply line diameter increase; larger capacity of the fuel filter; larger feed pump capacity (with parasitic power and heat dissipation); and control valve (dump or inlet metering) size and its electric requirements.
It is, accordingly, an object of the present invention to provide a high pressure common rail fuel supply pump, that is optimized for gasoline injection. In particular, it is an object to provide such a fuel supply pump, in conjunction with a conventional electric gasoline feed pump.
It is another object to provide such a gasoline supply pump, which is resistant to the formation of vapor cavities.
It is further object of the invention to provide such a high pressure supply pump which can maintain a constant rail pressure during the full rotation of the pump drive shaft, thereby facilitating direct open loop injector control.
It is yet another object of the invention, to provide a method of operating a common rail gasoline fuel injection system for a multi-cylinder internal combustion engine, in which direct open loop injector control is achieved without a high pressure accumulator external to the pump.
It is yet another object, to provide a high pressure gasoline fuel supply pump which is compact and produces low hydraulic noise.
A still further object is to provide a gasoline fuel supply pump in which variable rail pressure is achieved by a servo dump valve controlled by a proportional valve.
Another object is to provide a high pressure gasoline supply pump, in which a very efficient sealing arrangement prevents leakage from a very compact pump housing.
Yet another object is to provide a gasoline supply pump which can be mounted directly on a fuel tank so as to draw a fuel feed flow from the tank without the need for a distinct feed pump.
A further object of the invention is to provide a plunger plug for mounting in the housing to receive and guide a reciprocating plunger, which is easy to manufacture and install.
According to one fundamental aspect of the present invention, individual pumping plunger bores and associated pumping chambers are equi-angularly spaced and radially mounted in a pump housing. The pumping plungers are actuated radially outwardly and withdraw inwardly by an eccentric rotated by the pump drive shaft and associated captured sliding shoes. Because the shoes are forced to follow the eccentric over the full 360xc2x0 of rotation, the shoes themselves can play an integral role for implementing the function of an inlet check valve which controls flow through a charging passage in each plunger in a radial outward direction, to a respective plunger pumping chamber. During the radially inward movement of each plunger, whereby the plunger is drawn by the drive member and shoe toward the center of the pump, a vacuum is drawn at the pumping chamber. Relatively low pressure fuel in the pump cavity surrounding the drive member, is drawn through openings in the radially inner end of the plunger, through an inlet passageway in the plunger, and into the pumping chamber. The path which low pressure fuel follows from the cavity into the inlet passageway of the plunger, can be implemented in a variety of ways, including direct flow from a radially inner side wall of the plunger into the central inlet passageway; flow through a slot in the drive member which registers with a hole in each shoe and which in turn is in fluid communication with the inlet passageway in the plunger; or the retention of the shoes against the drive member can permit slight separation between a shoe and the drive member momentarily, to allow low pressure fuel to enter a hole in the foot of the shoe, which in turn is in fluid communication with the inlet passageway in the plunger. A common rail is preferably situated within the housing and fluidly connected to all the discharge passages from the pumping chambers, downstream of the discharged check valves.
Another aspect of the present invention, involves various arrangements for establishing a seal between the drive shaft and the cavity at feed pressure, from which fuel is drawn into the pumping chamber, to prevent leakage of fuel along the drive shaft and therefore from the pump housing. This is achieved in various embodiments, by having either a plurality of seal chambers in which the outermost chamber has a fluid connection to, e.g., the fuel tank, or in another embodiment, by providing a virtual seal at a thrust plate forming a boundary of the cavity, such that an adjacent seal chamber will be maintained at low pressure for connection to the fuel return line to the fuel tank.
In another aspect of the invention, a novel plunger plug arrangement is secured to the pump body, for providing the plunger bore, mounting the discharge check valve, and establishing a discharge passage, utilizing only two unitary components, each of which can be machined fully during one chuck set-up.
In yet another aspect of the invention, the high pressure gasoline fuel supply pump housing, particularly the body, also forms the housing for an electric motor unit, whereby the pump and motor unit can be mounted at the fuel tank. This takes advantage of the ability of the pump to draw fuel from the pump cavity through the plunger into the pumping chamber directly, or virtually directly, from the fuel tank, in some cases without the need for a primary or feed pump.